This invention relates to a stretchable composite sheet and process for forming the same with a plurality of gathers.
A stretchable sheet of well known art is generally classified into a sheet which is perse elastically stretchable and a stretchable composite sheet comprising a substantially non-stretchable sheet member and a plurality of stretchable elastic members bonded in a stretched state to this substantially non-stretchable sheet member. An example of conventional stretchable composite sheets will be explained as follows with reference to a drawing in FIG. 10 of the accompanying drawings. In FIG. 10, a transverse direction is indicated by an arrow X and a longitudinal direction is indicated by an arrow Y.
A composite sheet 40 comprises a pair of substantially non-stretchable fibrous nonwoven fabric 41, 42 placed upon each other and a plurality of stretchable elastic members 43 each extending in the transverse direction and spaced one from another by a given spacing in the longitudinal direction. These elastic members 43 are interposed between layers of the nonwoven fabric 41, 42 and bonded in tension in the transverse direction to the layers of the nonwoven fabric 41, 42. The composite sheet 40 is formed with a plurality of irregularly shaped gathers 44 and wrinkles 45 as the elastic members 43 contract. In this composite sheet 40 of well known art, these irregularly shaped gathers 44 and wrinkles 45 are:smoothed when the composite sheet 40 is placed under a tensile force in the transverse direction, and consequently the composite sheet 40 can be stretched in the transverse direction.
This composite sheet 40 includes regions 40a in which the gathers 44 and the wrinkles 45 are densely formed and regions 40b in which the gathers 44 and the wrinkles 45 are sparsely formed. In the regions in which many gathers 44 and wrinkles 45 are formed, a rate of elongation in the transverse direction in the composite sheet 40 presents a relatively high but correspondingly a layer force of elongation is high tensile force is required to smooth these gathers 44 and wrinkles 45. In the regions 40b in which the gathers 44 and the wrinkles 45 are sparsely formed, these gathers 44 and wrinkles 45 can be smoothed under a lower force of elongation than that in the regions 40a but the elongational rate of the composite sheet 40 in the transverse direction is lower than that in the regions 40a. In consequence, it is impossible to stretch this composite sheet 40 of well known art over its entire area in the longitudinal direction to the same length in the transverse direction by applying substantially uniform force of elongation.